Method for controlling a quantity of medium transferable between two rollers

ABSTRACT

A method for controlling a quantity of medium transferable from a screen roller of a printing machine onto a roller that is in contact with the screen roller includes exerting an influence upon a difference in circumferential speed between the screen roller and the roller in contact therewith, and further includes controlling the difference in the circumferential speed as a function of the printing speed of the printing machine, so that printed medium density remains at least approximately constant at least within a wide printing speed range.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method for controlling a quantity of mediumtransferable between two rollers, such as from a screen roller of aprinting machine onto a roller in contact with the screen roller.

So-called short inking units, such as anilox inking units, for example,which are installed in printing machines, have become known heretoforefrom the published German Patent Document DE 198 40 613 A1. Theycomprise a screen roller, also known as an anilox roller, which isformed on the circumference thereof with depressions capable of beingfilled with ink or varnish. The screen roller has a chambered doctorblade assigned thereto which wipes or scrapes the ink or varnish off thescreen roller. The screen roller co-operates with an ink applicatorroller, onto which a constant quantity of ink/varnish is transferred dueto the depressions. In the event of an increase in the printing andmachine speed, respectively, a slight decrease in the ink densitymeasured on a print carrier occurs because of the normallyhigh-viscosity offset ink. This may result from the fact that thedepressions are no longer filled correctly at higher printing speeds,because the depressions are no longer emptied so effectively uponcontact with the ink applicator roller or because the transfer of inkfrom the screen roller onto the ink applicator roller and, via a platecylinder, onto a blanket cylinder and from the latter onto the printcarrier is impaired.

In order to have an effect upon the printed ink density, also called theoptical density, in anilox inking units, it has become known heretoforeto exert an influence upon the slip between the screen roller and theink applicator roller. When the two rollers have the samecircumferential speeds, i.e., there is no slip, an optimum transfer ofink from the screen roller onto the ink applicator roller takes place.When slip occurs between these rollers, the printed ink densitydecreases due to the diminishing ink quantity transferred from thescreen roller onto the ink applicator roller.

It has become known from the hereinaforementioned published GermanPatent Document DE 198 40 613 A1 that it is unimportant whether the slipis positive or negative, and that only the absolute size thereof iscritical. By an adjustment of the slip, therefore, the printed inkdensity can be changed relatively quickly.

SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide a method of thetype mentioned in the introduction hereto, wherein, even at differentprinting speeds, a preferably uniformly good transfer of ink between therollers is realizable.

With the foregoing and other objects in view, there is provided, inaccordance with the invention, a method for controlling a quantity ofmedium transferable from a screen roller of a printing machine on aroller that is in contact with the screen roller, which comprisesexerting an influence upon a difference in circumferential speed betweenthe screen roller and the roller in contact therewith, and furthercomprises controlling the difference in the circumferential speed as afunction of the printing speed of the printing machine, so that printedmedium density remains at least approximately constant at least within awide printing speed range.

In accordance with another mode, the medium controlled by the method isa medium selected from the group thereof consisting of ink and varnish.

In accordance with a further mode of the method, the difference incircumferential speed is zero at a standard printing speed.

In accordance with an alternative mode of the method, the difference incircumferential speed is zero at a printing speed higher than a standardprinting speed.

In accordance with an added mode, the method further comprisesdetermining, for the difference in the circumferential speed dependentupon the printing speed, a characteristic curve at which the printedmedium density remains constant.

In accordance with an additional mode, the method further comprisesstoring the characteristic curve in a control device.

In accordance with yet another mode, the method further comprisescontrolling the difference in the circumferential speed as a function ofa circumferential speed of a cylinder selected from the group thereofconsisting of a printing-form cylinder and a blanket cylinder capable ofbeing supplied with the medium by the screen roller.

In accordance with yet a further mode, the method further comprisesincreasing the temperature of the screen roller so as to raise theprinted medium density.

In accordance with a concomitant mode, the method further compriseslowering the temperature of the screen roller so as to reduce theprinted medium density.

Accordingly, the quantity of medium, which is capable of beingtransferred from a screen roller of a printing machine onto a roller,for example an ink applicator roller, which is in contact with thescreen roller, is controlled by exerting influence on a difference incircumferential speed between the screen roller and the roller incontact therewith. The method is distinguished in that the difference incircumferential speed is controlled as a function of theprinting/machine speed of the printing machine, so that the printedmedium density remains constant or approximately constant at leastwithin a wide printing speed range. This makes it possible to ensurethat the transfer of medium from the screen roller onto the succeedingor following roller is uniformly good at virtually all printing speeds.

In connection with the invention of the instant application, by the“printed medium density” there is meant the density of a printed imagetransferred onto a print carrier. It is also known as optical density.Consequently, “printed medium density” does not refer to the materialdensity of the printing medium.

The printing machine may be a sheet-fed or web-fed machine which isoperated in wet offset or dry offset. The medium is preferably liquid,but may also be pasty, and is preferably an ink or a varnish.

In a preferred embodiment, provision is made for the circumferentialspeed difference to be zero at a standard printing speed and preferablyat a printing speed higher than the standard printing speed. At thestandard printing speed, therefore, the screen roller and the rollerco-operating with the latter run synchronously. The standard printingspeed is the speed at which the printing machine mainly operates. It mayamount, for example, to 2.5 m/s in the case of a sheet-fed press and to9 m/s in the case of a web press. In this mode of the method, it isassumed that, in the speed range lying above the standard printingspeed, the printed medium density is no longer kept exactly constant.Here, however, instead, the printing unit can also operate most of thetime without slip between the rollers. The wear of the rollers iscorrespondingly low here. A circumferential speed difference between thescreen roller and the succeeding or following roller occurs here too,while the printing machine is being set up, i.e., during thecommencement of the run-off of a printing order and while the printingmachine is accelerated to the standard printing speed. This period oftime is relatively short, when compared with the duration of the run-offof the entire printing order.

A method can also be realized readily wherein the circumferential speeddifference is zero at the maximum printing/machine speed. In this case,although the printed medium density would be capable of being set at aconstant level at all printing speeds lower than the maximum printingspeed, most of the time there would nevertheless be slip between thescreen roller and the succeeding or following roller, with the resultthat the useful life of the rollers is reduced.

Furthermore, a mode of the method is preferred, wherein a characteristiccurve at which the printed medium density remains constant is determinedfor the circumferential speed difference dependent upon the printingspeed. The characteristic curve may be determined, for example, bytests, in that the slip necessary between the screen roller and thesucceeding or following roller so that the printed medium densityremains constant is detected for various printing speeds. A continuouscharacteristic curve can be determined from these values byextrapolation and stipulates for each printing speed a new slip value(circumferential speed difference) and, for each slip value, theprinting speed which is necessary for this purpose, respectively, atwhich the printed medium density is constant. In connection with theinvention of the instant application, the term “characteristic curve”also refers to a function table which specifies discrete slip values forvarious printing speed ranges. One and the same slip value thereforeapplies to different printing speeds here, i.e., the printed mediumdensity is not always exactly constant within this printing speed range,but these very slight density differences influence the print result toonly a harmless extent.

In an advantageous mode, provision is made for storing thecharacteristic curve in a control device. Depending upon the printcarrier, which may be formed, for example, of paper, cardboard, plasticmaterial or metal, and upon the type of ink or varnish, thecharacteristic curve may be different. With the aid of the controldevice, the characteristic curve provided for the respective printcarrier and the ink/varnish, respectively, is employed in order to adaptthe slip (circumferential speed difference) between the screen rollerand the succeeding or following roller to various printing speeds, insuch a way that the printed ink density and the varnish density,respectively, is constant at any printing speed lower than the standardprinting speed.

Other features which are considered as characteristic for the inventionare set forth in the appended claims.

Although the invention is illustrated and described herein as embodiedin a method for controlling a quantity of medium transferable betweentwo rollers, it is nevertheless not intended to be limited to thedetails shown, since various modifications and structural changes may bemade therein without departing from the spirit of the invention andwithin the scope and range of equivalents of the claims.

The construction and method of operation of the invention, however,together with additional objects and advantages thereof will be bestunderstood from the following description of specific embodiments whenread in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevational diagrammatic and schematic view of anexemplary embodiment of a printing machine;

FIG. 2 is a plot diagram or graph, wherein printing/machine speed isplotted on the abscissa axis, and slip between a screen roller and asucceeding roller is plotted on the ordinate axis; and

FIG. 3 is a plot diagram or graph, wherein printing/machine speed isplotted on the abscissa axis, and printed ink density is plotted on theordinate axis.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the figures of the drawings and, first, particularly toFIG. 1 thereof, there is shown therein a diagrammatic and schematic viewof an exemplary embodiment of a printing machine 1 comprising a printingunit 3 and an inking unit 5.

The inking unit 5 is formed here of a short inking unit, more preciselyan anilox short inking unit, and comprises a screen roller 7, also knownas an anilox roller, into the circumferential surface of whichdepressions 9 are introduced, for example cells or grooves, forreceiving a liquid medium therein. It is assumed hereinafter, purely byway of example, that the medium is a liquid ink. The inking unit 5 has,furthermore, a chambered doctor blade 11, with the aid of which the inkis introduced into the depressions formed in the screen roller 7, andthe circumferential surface of the screen roller is subsequently wipedor scraped off. The chambered doctor blade 11 is supplied with ink via aline 15 connected to a reservoir 13. The ink is pumped out of thereservoir 13 to the chambered doctor blade 11 with the aid of a pump 17.

The inking unit 5, furthermore, has a roller 19 which co-operates withthe screen roller 7 and is formed, for example, as an ink transferroller 19 with the rubber-elastic casing. The term “co-operate” has themeaning, here, that the screen roller 7 and the roller 19 are in contactwith one another and form a roller nip.

A printing form 23, here formed by a plate cylinder 21, co-operates withthe roller 19 and is itself in contact with a blanket cylinder 25. Aprinting image is applied by the latter to a print carrier 27, forexample a sheet or web.

The roller 19, the plate cylinder 21 and the blanket cylinder 25 areconnected to one another in a conventional manner via non-illustratedgearwheels of a drive transmission and are driven at the samecircumferential speeds (circumferential speed difference/slip=0).

The roller 19 and the plate cylinder 21 are of the same diameter.

In the exemplary embodiment illustrated in FIG. 1, provision is made forthe circumferential speed of the screen roller 7 to be capable of beingset individually in relation to the roller 19, so that a slip of between0% and 10% is realizable. As indicated in FIG. 1, this may take placewith the aid of a specific motor drive 29 for the screen roller 7.Alternatively thereto, a variable-speed gear transmission may beprovided, the drive of which takes place for the most part via agearwheel connection to the roller 19 and wherein only the differentialcircumferential speed between the screen roller 5 and the roller 19 isadditionally coupled by a comparatively small motor. A third alternativeis to install an adjustable mechanical gear transmission. Furtherconstructions for realizing a circumferential speed difference betweenthe screen roller 7 and the roller 19 are possible.

In the exemplary embodiment illustrated in FIG. 1, therefore, the screenroller 7 is equipped with an individual drive. Furthermore, a speedtransmitter 31 arranged on the blanket cylinder 25 is provided, whichcommunicates the then-current printing/machine speed via a signal line33 to a diagrammatically illustrated control device 35. Alternatively,the signal for the then-current printing/machine speed may also comedirectly from a non-illustrated main drive motor of the printing machine1 and from the printing unit 3, respectively.

The control device 35 stores a characteristic curve, also known as arun-up curve, which stipulates the necessary circumferential speeddifference between the screen roller 7 and the roller 19 as a functionof the then-current printing speed (V_(M)) at which the printed/opticalink density remains constant. The appertaining slip of the screen roller7 is therefore retrieved from the characteristic curve, and then thecorrected speed (V₇) for the drive of the screen roller 7, i.e., themotor drive 29 connected to the control device 35 via a signal line 37,is stipulated or prescribed.

FIG. 2 is a plot diagram or graph, wherein, as a percentage, theprinting/machine speed v is plotted on the abscissa axis and the slip s,i.e., the circumferential speed difference between the screen roller 7and the succeeding or following roller 19 is plotted on the ordinateaxis. In the graph, a curve 39 is depicted, which indicates, for eachprinting speed, the required circumferential speed difference betweenthe screen roller 7 and the roller 19, so that the optical density ofthe ink to be transferred between the rollers 7 and 19, and of theprinting image printed onto the print carrier, respectively, ispreferably constant, but is at least approximately constant within theframework of a narrow tolerance.

It is apparent that the slip is relatively high at a low printing speedv, and decreases with a rising printing speed v, until it finallyapproaches zero and is zero, respectively, at a standard printing speedv_(n). The standard printing speed is the speed at which the printingmachine mainly operates. Even in the event of a further increase in theprinting speed to the maximum printing speed v_(max), the slip s remainsunchanged at zero. When the circumferential speed difference between thescreen roller 5 and the roller 19 is controlled along the characteristiccurve 39, which may readily be performed with the aid of the controldevice 35, a constant optical ink density is realizable in the rangebetween the minimum printing speed and the standard printing speedv_(n).

FIG. 3 shows a graph wherein the printing/machine speed v is plotted onthe abscissa axis, and the optical density D_(v) of the ink to betransferred from the screen roller 7 onto the roller 19 is plotted onthe ordinate axis. An unbroken line 41 represents the profile of theoptical density, such as occurs when the slip between the rollers 7 and19 is regulated or controlled in a way described with reference to FIG.2. It becomes clear that the optical density is constant up to thestandard printing speed v_(n) and falls a little in the speed rangelying thereabove, up to the maximum printing speed v_(max). The reasonfor this is that the slip s remains zero even for printing speeds higherthan the standard printing speed. The comparison, a broken line 43,represents the profile of the optical density against the printing speedv if slip regulation were not carried out, i.e., if the slip s were, forexample, zero at every printing speed; the optical density D_(v)decreases continuously with an increasing printing speed v.

As is apparent from FIG. 3, the ink density level achieved by thecircumferential speed difference controller according to the inventionis below that when slip regulation, such as is described with referenceto FIG. 2, is not carried out. By an increase in the temperature of thescreen roller 7, however, it is possible to raise the optical densitycontinuously again, as indicated by the broken line 41′. Of course, itis also possible, by reducing the screen-roller temperature, to lowerthe optical density D_(v) continuously, as indicated by the broken line41″.

All the varying modes of the method have in common the fact that theslip s, i.e., the circumferential speed difference between the screenroller 7 and the ink applicator roller 19, is stipulated or prescribedby the characteristic curve 39 for each printing speed v, so that heoptical density D_(v) is constant at all printing speeds v lower thanthe standard printing speed v_(n). Insofar as the characteristic curve39 is stored in the control device 35, action by the operating personnelin order to set the required circumferential speed difference,respectively, is preferably not required, at most, for manual finesetting.

1. A method for controlling a quantity of medium transferable from ascreen roller of a printing machine onto a roller that is in contactwith the screen roller, which comprises: exerting an influence upon adifference in circumferential speed between the screen roller and theroller in contact therewith, and further comprises controlling thedifference in the circumferential speed as a function of the printingspeed of the printing machine, so that printed medium density remains atleast approximately constant at least within a wide printing speedrange; and determining, for the difference in the circumferential speeddependent upon the printing speed, a characteristic curve at which theprinted medium density remains constant, and storing the characteristiccurve in a control device.
 2. The method according to claim 1, whereinthe medium controlled thereby is a medium selected from the groupthereof consisting of ink and varnish.
 3. The method according to claim1, wherein the difference in circumferential speed is zero at a standardprinting speed.
 4. The method according to claim 1, wherein thedifference in circumferential speed is zero at a printing speed higherthan a standard printing speed.
 5. The method according to claim 1,which further comprises determining, for the difference in thecircumferential speed dependent upon the printing speed, acharacteristic curve at which the printed medium density remainsconstant.
 6. The method according to claim 3, which further comprisesstoring the characteristic curve in a control device.
 7. The methodaccording to claim 1, which further comprises controlling the differencein the circumferential speed as a function of a circumferential speed ofa cylinder selected from the group thereof consisting of a printing-formcylinder and a blanket cylinder capable of being supplied with themedium by the screen roller.
 8. The method according to claim 1, whichfurther comprises increasing the temperature of the screen roller so asto raise the printed medium density.
 9. The method according to claim 1,which further comprises lowering the temperature of the screen roller soas to reduce the printed medium density.